Tachyarrhythmias are abnormal heart rhythms characterized by a rapid heart rate. Tachyarrhythmia generally includes supraventricular tachyarrhythmia and ventricular tachyarrhythmia. Fibrillation is a form of tachyarrhythmia further characterized by an irregular heart rhythm. In a normal heart, the sinoatrial (SA) node, the heart's predominant natural pacemaker, generates electrical impulses, called action potentials, that propagate through an electrical conduction system to the atria and then to the ventricles of the heart to excite the myocardial tissues. The atria and ventricles contract in the normal atrio-ventricular sequence and synchrony to result in efficient blood-pumping functions indicated by a normal hemodynamic performance. Ventricular tachyarrhythmia occurs when the electrical impulses propagate along a pathologically formed self-sustaining conductive loop within the ventricles or when a biologic pacemaker (focus) in a ventricle usurps control of the heart rate from the SA node. When the atria and the ventricles become dissociated during ventricular tachyarrhythmia, the ventricles may contract before they are properly filled with blood, resulting in diminished blood flow throughout the body. This condition becomes life-threatening when the brain is deprived of sufficient oxygen supply. Ventricular fibrillation (VF), in particular, stops blood flow within seconds and, if not timely and effectively treated, causes immediate death. In very few instances a heart recovers from VF without treatment.
Ventricular cardioversion and defibrillation are used to terminate most ventricular tachyarrhythmias, including ventricular tachycardia (VT), and VF. An implantable cardioverter defibrillator (ICD) is a CRM device that delivers cardioversion/defibrillation pulses, each being an electric shock, to terminate a detected tachyarrhythmia episode by depolarizing the entire myocardium simultaneously and rendering it refractory. An ICD typically also delivers another type of electrical therapy for tachyarrhythmia known as anti-tachycardia pacing (ATP). In ATP, the heart is competitively paced in an effort to interrupt the reentrant loop causing the tachyarrhythmia.
An ICD typically includes pacing and cardioversion/defibrillation capabilities. In addition to anti-tachyarrhythmia therapies including ATP and cardioversion/defibrillation, the ICD may also deliver chronic pacing therapies such as bradycardia pacing therapy and cardiac resynchronization therapy. A detected tachyarrhythmia episode triggers several responses in the ICD, such as classification of the tachyarrhythmia episode and delivery of one or more anti-tachyarrhythmia therapies based on the classification, and termination or adjustment of bradycardia pacing therapy or cardiac resynchronization therapy. Such responses are typically functional features of the ICD that are controllable by a user such as a physician or other caregiver. While patients are potentially benefiting from increased functional features of modern ICDs, there is a need for simplifying the programming of these features to facilitate their proper utilization.